This is true because political science and actual participation in politics are very different things. Anthropologists can study primitive tribes without being members of those tribes. Economists can study the behavior of companies without actually running a company. It is just the same with political scientists. Political scientists study the workings of the political system and the people who participate in that system. That does not mean that they have to be active members in the system.
Political scientists are academics. They study things. They try to understand the political system. Many of them study very narrow aspects of the political system such as what factors can be statistically proven to affect the way members of Congress vote. This means that they need certain skills. Their skills need to be academic and intellectual.
Politicians are not academics. They try to get elected and they try to make laws. They are not interested in studying narrow aspects of the political system. Instead, they are interested in how they can get their bills passed and how they can get reelected. This means they need more in the way of people skills and public speaking skills.
In short, politicians and political scientists need different skill sets because they do different things. Therefore, the two groups do not typically overlap.